Thank you Dave!
This seems to confirm my gut feeling. I suspect if the ferrite were
almost solid with just a small hole through the center (just larger
than the wire) Z (or more importantly equivalent series resistance)
would increase dramatically. This suggests that at VHF using a
ferrite with an ID significantly larger than the wire passing
through it *might* not be optimum. It may (or may not) be that using
a smaller ferrite which more closely fits the wire is better even
though from the data sheet its Z and Rs may be less than the larger
part. It may (or may not) be that 2-4x ratio of wire size to ferrite
ID is not enough to have a significant effect. My point is that
there are some unknowns here. I have spent a great deal of time
looking for understandable data on this and have come up with none.
I think it would be very helpful to the VHF community if someone
with the proper equipment could look into this and publish some
curves like those for multi-turn HF chokes in K9YC's excellent
RFI-Ham.pdf paper. Z and Rs at VHF vs. wire diameter for a few
common sizes of ferrite, for example. If I had a VNA and the
expertise to use it correctly I would take on this project, but I don't.
I may be able to provide an experiment. Given the unknowns I am
unwilling to risk buying only a large quantity of just one (large)
size ferrite. I will buy a large quantity of two sizes that will be
a close fit for many of the cables I need to deal with. For the time
being I will buy a relatively small quantity of a smaller size which
would be a close fit for my smallest cables, as these are the ones
where I question the best choice. Assuming I find a small cable upon
which installation of ferrites reduces my receive noise, I will
compare noise reduction with three differnet size ferrites on that
cable: one that is a close fit, one that is a loose fit, and one
that is even more loose. This may at least tell me whether I am
better off buying the smaller ferrites in quantity or going with
more of the larger size.
73,
Paul N1BUG
On 08/21/2017 08:10 AM, David Robbins wrote:
I scanned a few papers this morning that talk about things like the ferrite
window fill factor and variations of inductance with wire size... then
pulled out my vna and a ferrite to try a quick experiment. My conclusion is
that if the ferrite is much larger in diameter than the wire going through
it the inductance will be decreased. My quick explanation of this is that a
current through a wire generates a magnetic field around the wire that is
strongest near the wire and gets weaker the farther away you go. To get the
most flux in the ferrite in order for it to increase the inductance the
ferrite must be close to the surface of the wire. my simple test with the
vna used a 22ga clip lead going once through a 2" diameter ferrite, when the
wire was in the middle of the core the |Z| of the single turn coil was like
38 ohms, when it was touching the inside edge it increased to 42 ohms, so it
appears to be a real and probably not insignificant effect.
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